As is well known, fresh cut lumber includes a large quantity of moisture and it has been the practice to subject such lumber to heat treatment to drive off the moisture so that the lumber, as sold, will be less subject to warping or bending during storage or after installation in a structure.
Drying lumber is typically performed in a batch kiln process, where an insulated chamber is used that is adapted to control several drying process conditions, including, but not limited to air temperature in the kiln, air speed across the lumber, and the relative humidity in the chamber. As these kilns are a closed atmosphere, packages of sawn lumber, often referred to as green lumber, separated by stickers are placed in the kiln in batches. The packages are often loaded vertically, horizontally, and end to end.
Once the batch of packages are in place, the chamber is closed and a schedule or recipe of temperatures and relative humidity is initiated for a determined time interval or until a certain moisture content in the lumber is achieved. Generally, the schedule gradually increases the temperature in the chamber and lowers the relative humidity. This allows the lumber to give up its moisture to the surrounding air, which may then be vented to the outside atmosphere.
The particular schedule used and the drying time varies depending on a number of factors, including, but not limited to, lumber type/species, thickness, moisture content, end use of the lumber and the like. Once the schedule has run, the kiln doors are opened and the packages are removed from the kiln chamber and further prepared for shipping to a final destination. This opens the chamber to atmospheric conditions and can often require a significant amount of time and energy to bring the next charge of green lumber up to drying conditions.
While lumber is typically dried as fast as possible depending on the cell structure, drying too rapidly can have adverse effects on the lumber, such as checking, splitting, warping, cupping, and the like. Accordingly, the temperature and humidity in the kiln, as well as the drying time will vary depending on the above listed factors. For example, Red Oak may take up to 28 days dry from green to 7% moisture content, while Southern Yellow Pine can be dried in approximately 20-24 hours from green to 15% moisture content.
As described in U.S. Pat. No. 7,963,048 (Pollard), U.S. Pat. No. 8,201,501 (Tinsley), and U.S. Pat. No. 8,342,102 (Tinsley), the complete disclosures of which are incorporated by reference herein, disclose the use of two generally parallel paths for separate continuous drying lines which are moved in opposite directions through a plurality of chambers which are aligned along the separate paths and in communication with one another to increase the efficiency of the drying process. This is accomplished by continuously moving loads of lumber while conserving the heat that builds up in each load as it is continuously moved along one path in the kiln and transferring some of that heat from a load of dried lumber to an incoming load of green lumber moving from the opposite end of the kiln as they pass one another in the kiln.
Prior art dual path kilns utilize internal reversible propeller fans because the flow of air through the lumber in central heating chamber must be periodically reversed, typically every three hours. Many of the prior art kilns also reverse the flow of air through the lumber in each of the end chambers. While the air flow through the lumber is being reversed, the lumber is not being dried. Furthermore, reversible fans are mechanically inefficient compared unidirectional fans. Moreover, since the fans are inside the hot central chamber they are fully exposed to a harsh environment.
There is a need for a continuous kiln that does not have the losses in productivity associated with fan reversals and can be used continuously to dry the lumber. There is also a need for a continuous kiln that can use single direction fans.